16. Scanner characterization and calibration - 2011

16. Scanner characterization and calibration - 2011 -...

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Lecture 16 Scanner Characterization and Calibration - Sanjyot Gindi M.S.E.C.E, Purdue University July 18th 2008
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Masters Thesis Presentation, July 18th, 2008 Sanjyot Gindi Slide 2 of 43 Objectives: Spectral model based characterization of Samsung SCX5530 scanner. Empirical or regression based characterization of Samsung, HP Photosmart and Epson Photo RX700 scanners. Plotting of color gamuts and comparison of the 3 scanners based on them.
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Masters Thesis Presentation, July 18th, 2008 Sanjyot Gindi Slide 3 of 43 Basic working of a scanner Lamp Glass-top Paper to be scanned ‘Target’ is placed on the glass top A movable scanner-head consists of a lamp and sensors Light from the lamp incident on the target is reflected back to the scan head. This light passes through an optical assembly and is received by sensors. RGB values of the color of target – ‘device-dependent’ color space.
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Masters Thesis Presentation, July 18th, 2008 Sanjyot Gindi Slide 4 of 43 Modeling of a scanner system Understanding the color characteristics and consistently predicting the color values. Two steps : Calibration: Linearization or gray balancing Characterization: Transformation from device dependent RGB values to co-ordinates in the device independent color space like CIE XYZ, L*a*b*. [1]
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Masters Thesis Presentation, July 18th, 2008 Sanjyot Gindi Slide 5 of 43 Methods of characterization 1. Regression based method: Considers the scanner system as a ‘black box’ Mapping from linear RGB to CIE XYZ 2. Model based method: Uses known spectral functions of the components of a scanner system.
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Masters Thesis Presentation, July 18th, 2008 Sanjyot Gindi Slide 6 of 43 1. Regression based method Transformation Matrix T NL R NL G NL B R H G B R l G l B l X Y Z R , G , B are the values obtained from the scanner with gamma on, shading on. Rl, Gl, Bl are the linearized values X,Y,Z values are the CIE XYZ values obtained using the X-Rite Spectrophotometer with D65 illuminant. NL represents the non-linear relationship between output R , G , B values and Rl, Gl, Bl values
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Masters Thesis Presentation, July 18th, 2008 Sanjyot Gindi Slide 7 of 43 Calibration procedure: ‘Gray balancing’ using Y (Luminance) values of the neutral gray patches on the Kodak Q60 target. Linear R,G,B values obtained using a power-law curve fit given by: , where R l = Y.
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Masters Thesis Presentation, July 18th, 2008 Sanjyot Gindi Slide 8 of 43 Gray Balance curves X axis : R,G,B values from Scanner (0-255) Y axis: (Y) Luminance values from X-Rite (0-100) a = 79.63 b = 2.272 c = 0.5905 a = 81.58 b = 2.033 c = 1.344 a = 84.14 b = 2.198 c = 1.093
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Sanjyot Gindi Slide 9 of 43 Characterization: Find scanner RGB values for Kodak Q60 target : 240 color patches CIE XYZ values of color patches using X-Rite spectrophotometer for D65 illuminant condition. Determine the 3x3 transformation matrix T given by:
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This note was uploaded on 02/19/2012 for the course ECE 638 taught by Professor Staff during the Fall '08 term at Purdue University-West Lafayette.

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16. Scanner characterization and calibration - 2011 -...

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